1. Field of the Invention
The present invention relates to an impedance matching apparatus to be interposed between a high-frequency power source and a load, for matching the impedance of the high-frequency power source and the impedance of the load.
2. Description of the Related Art
Manufacturing process of semiconductors or flat panel displays includes a plasma process. Some of plasma process chambers to be employed in the plasma process require to be applied to high-frequency voltage with a radio frequency band, for example, from 100 kHz to 300 MHz.
Between the high-frequency power source and the plasma process chamber acting as the load, an impedance matching apparatus is interposed. The impedance matching apparatus serves to match the impedance of the high-frequency power source and that of the plasma process chamber i.e. the load, to minimize the reflected power from the load to the high-frequency power source, thus maximizing the power supply to the load.
FIG. 10 depicts a configuration of a high-frequency power supply system including an impedance matching apparatus, disclosed in JP-A H05-63604. The impedance matching apparatus according to the cited document, has an input terminal connected to a high-frequency power source 41, and an output terminal connected to a load 42. The impedance matching apparatus includes a matching circuit 43 including an input-side detector 44, inductors L2, L3 and variable capacitors VC3, VC4 serving as impedance variable devices.
The input-side detector 44 detects a high-frequency voltage V and a high-frequency current I with a radio frequency band, and a phase difference θ between the high-frequency voltage V and the high-frequency current I. The detected high-frequency voltage V, high-frequency current I and the phase difference θ there between are input to a computer 46 via an A/D converter 45 separately provided from the impedance matching circuit 43.
The computer 46 calculates an input impedance Zi of the impedance matching circuit 43, i.e. the impedance Zi present in the impedance matching circuit 43 in a direction from the input terminal 43a toward the load 42, based on the results detected by the input-side detector 44 (i.e. high-frequency voltage V, high-frequency current I, phase difference θ).
Each of the variable capacitors VC3, VC4 includes an adjustment unit (not shown) to change the capacitance of the variable capacitor VC3 and VC4, respectively, controlled by a motor M when a control signal output from the computer 46 is input to the driving voltage supplier 47 so as to drive the motor M. The computer 46 detects the adjustment positions of the variable capacitors VC3, VC4, to thereby calculate the impedances Zc3, Zc4 of the variable capacitors VC3, VC4 serving as the impedance variable devices.
The computer 46 calculates a load circuit-side impedance Zo at the output terminal 43b of the impedance matching circuit 43 in a direction toward the load 42, based on the input impedance Zi and the impedances Zc3, Zc4 of the impedance variable devices.
The computer 46 varies the adjustment positions of the variable capacitors VC3, VC4 so that the input impedance Zi matches with the output impedance Zp (for instance, 50Ω) on the side of the high-frequency power source 41 based on the calculated load circuit-side impedance Zo, thus matching the impedance of the high-frequency power source 41 and that of the load 42.
The impedance matching circuit 43 according to the cited document acquires the load circuit-side impedance Zo based on the input impedance Zi calculated from the high-frequency voltage V, the high-frequency current I, and the phase difference θ therebetween detected by the input-side detector 44, and on the impedances Zc3, Zc4 of the variable capacitors VC3, VC4 detected by the computer 46 with regard to the adjustment positions of the variable capacitors VC3, VC4, and then determines the adjustment positions of the variable capacitors VC3, VC4 to be matched.
When handling a high-frequency wave, however, the circuit devices serving as the matching circuit of the impedance matching circuit 43 include not only the variable capacitors VC3, VC4 and the inductors L2, L3, but also stray capacitance components between those parts and the housing and inductance components of copper plates or waveguides connecting those parts, and influences such impedance components is unable to disregard on the impedance matching performance.
The impedance controlling method according to the cited document employs the impedance matching circuit 43 consisting only of the variable capacitors VC3, VC4 and the inductors L2, L3, so as to determine matching characteristics of the matching circuit at the current adjustment positions, based on the impedance-values Zc3, Zc4 of the variable capacitors VC3, VC4 at the current adjustment positions, and on impedance values Z13, Z14 of the inductors L2, L3. Accordingly, from a strict viewpoint, the impedance components such as the stray capacitance are not counted in the matching circuit, and hence the matching circuit is not designed to perform in consideration of the impedance components such as the stray capacitance. As a result, the matching circuit according to the cited document fails to perform with sufficient accuracy, especially in a high-frequency region.
Also, the impedance components such as the stray capacitance readily change depending on the shape of the housing around the impedance matching circuit 43, or on the positional relation inside the impedance matching circuit 43 of the parts such as the variable capacitors VC3, VC4, the inductors L2, L3, and the other parts, and the wirings. Therefore, the matching circuits including the impedance components such as the stray capacitance may have different characteristics depending on the internal structure of the matching apparatus, even though the matching circuits are constituted of the same variable capacitors VC3, VC4 and the inductors L2, L3, and hence may suffer from the problem of unequal matching accuracy among the apparatuses.